Are Gut Bacteria Making You Fat? Part I: The Second Genome

The obesity epidemic has quickly become one of the greatest health challenges humans have ever faced, and the weight loss industry is now a billion dollar market that provides dieters with supplements, surgeries, and diet plans that promise instant fat loss. However, it hasn’t always been this way. We don’t have to go back more than a couple of decades to understand that the obesity epidemic has evolved over an extremely short period of time. The most recent estimates suggest that the number of overweight and obese people worldwide has risen from 857 million in 1980 to 2.1 billion in 2013, which represents an increase in both adults (28% increase) and children (47% increase) in the past 33 years (1). While it’s often believed that high-income developed nations have the highest obesity rates, several low-income countries actually surpass the stereotypical fat nation United States. Today, about 62% of the world’s obese population live in developing countries which have adopted the western dietary pattern (refined, calorie-dense, and highly palatable food) (1).

A person is considered obese when body mass index is 30 or over, a level of body fat accumulation that has negative effects on health. As excess fat tissue releases many inflammatory mediators and goes hand-in-hand with metabolic disturbances, hormonal dysregulation, and an altered gut microbiota, it’s no surprise that obesity is associated with an increased risk of several types of chronic disorders, such as type-2 diabetes, cardiovascular disease, and different types of cancers (2). Besides the obvious individual challenges that come with being obese, the obesity epidemic is also a major economic burden for the society at large.

However, the obesity epidemic hasn’t taken root everywhere. Some traditional populations which are still largely unaffected by the western lifestyle have low obesity rates, and among hunter-gatherers who still eat ancestral diets and live in an environment that closely resembles that of our paleolithic ancestors, obesity is virtually unheard of (3,4,5). This observation has led to the characterization of obesity as a disease of civilization, also sometimes called a mismatch disease.

This label as a mismatch disease is based on the idea that our genome was forged in the ancestral natural environment, an environment that demanded for a physically active lifestyle, periods of food shortage, diets rich in nutritious whole foods, and other factors that are common features of the hunter-gatherer lifestyle. When we divert too far away from this environment, as we’re now seeing in the industrialized world where we have easy access to cheap and palatable food, don’t need to expend a lot of energy to acquire food and shelter, and in general have disconnected ourselves from the natural human environment, disorders such as obesity emerge.

It’s generally accepted that obesity is caused by a combination of environmental and genetic factors, and few people with a general understanding of thermodynamics will disagree that the reason we gain fat is because we expend less energy than we consume. This is also consistent with studies which show that the rapid surge in obesity rates goes hand in hand with an increased energy intake (6). However, the fact that we’re now eating more food than we used to doesn’t really provide that many answers. The questions we should really be asking ourselves are; why are we eating more food than before, and why do some people gain more fat on a identical diet than others?

The human microbiome

There are many possible answers to the questions above, but during the last decade, more and more research is starting to show that the human microbiome – the collective genomes of all the microbes that live in and on the human body – plays an important role in regulating food intake, energy expenditure, and fat storage (7,8). This second genome can be considered both a genetic and environmental factor, since we know the microbiome to a certain extent is inherited from our mother, but it’s also shaped by diet, environment, and lifestyle throughout life.

The human microbiome is now one of the hottest research subjects in the scientific community, and thanks to massive science projects, such as the human microbiome projects (U.S.), MetaHit, and other scientific studies around the world, we now know that most of the cells in the human body are microbial and that the genetic repertoire of these microbes it at least hundred times greater than that of the human host (9,10).

The human microbiota – the collection of microorganisms that live in and on the human body – consists of both eukaryotic cells (e.g., fungus), prokaryotic cells (e.g., bacteria), and organisms that have no cellular structure (e.g., viruses). Trillions of microorganisms from thousands of different species are found in and on the human body, and each location has a unique bacterial makeup. (Click the image for larger picture)

The gut microbiome

While the microorganisms associated with the human body are found in various locations, such as the skin, lungs, and vagina, the largest bacterial communities are found in the gastrointestinal tract. Most of these gut microbes are found in the large intestine, but the upper GI tract also harbors complex bacterial communities. This gut microbiome – the collective genomes of all the microbes that live in the gut – plays an especially important role in human health. While it was previously believed that the role of these gut bacteria primarily was to digest indigestible carbohydrates, such as polysaccharides found in plants, we’re now learning that the role of the gut microbiome in human health stretches far beyond the breakdown of non-starch polysaccharides. The gut microbiota regulates our immune system, controls intestinal permeability, and even affects our mood and mental state (11,12).

In a healthy, balanced state, these gut microbes live silently in a mutualistic symbiosis with the human host. We provide the microbial communities with an environment in which they can live, and they provide us with essential functions that we can’t do without. However, if we perturb these microbial ecosystems we can end up changing the community structure, wiping out key species, and promoting a state of dysbiosis (microbial imbalances).

Researchers are now linking alterations in the gut microbiome to all sorts of different diseases, such as cancer, cardiovascular disease, acne vulgaris, obesity, and even psychological disorders such as autism and ADHD (11.13,14). While there’s still a way to go in terms of determining cause and effect in a lot of these disorders and the exact role of the microbiome, it’s clear that our microbial inhabitants play an essential role in our health and well-being.

In this series of posts I’ll look into the following question: What role does the human microbiome play in obesity?

About Author

Eirik is a nutrition student, personal trainer, and writer. He’s educated as a personal trainer from the Norwegian School of Sport Sciences and also has additional courses in sales/coaching, kettlebells, body analysis, and functional rehabilitation. Subscribe to his website OrganicFitness.com if you want to stay updated on his work.